In single-bar or whole-body impregnation, the generator winding bars of turbogenerators are shielded from cavities and detachments by an inner conducting layer (“inner potential grading” IPG) and an outer conducting layer (“external corona shielding” ECS). A turbogenerator is at present usually realized in the form of a three-phase AC synchronous machine comprising a solid two-pole or four-pole rotor. The power range of such a turbogenerator is typically between approximately 20 MVA and approximately 2000 MVA.
The stator of a conventional synchronous generator comprises a multiplicity of so-called stator windings, in which an AC voltage is induced by inductive interaction with the rotating rotor, to which a constant current is applied. The stator windings are accommodated in a so-called laminated core. This is used, inter alia, for guiding and intensifying the magnetic field. In order to reduce losses as a result of eddy currents, the entire laminated core is constructed from thin laminations which are insulated from one another. The stator windings consist of a multiplicity of bars, whose respective central pieces (the so-called “active parts”) are inserted into slots in the laminated core. The individual bars emerge from the slots in involute fashion at the so-called “end winding”. There, the individual bars are interconnected to form the stator winding (i.e. contact is made between said individual bars).
The bars or bar regions lying in the laminated core are at a high electrical potential and are therefore insulated from one another and from the earthed laminated core by a main insulating layer.
In order to avoid partial discharges at operating voltages of a few kV, the main insulating layer is generally shielded from cavities and detachments by an inner and an outer conducting layer (“inner potential grading” IPG and “external corona shielding” ECS). The electrical field strength in the main insulating layer is reduced starting from the IPG in a radial direction up to the ECS. This ensures that the electrical field remains only inside the main insulation and no partial discharges between the main insulation and an earthed laminated core take place. In addition, for axial grading of the electrical field at the end of the external corona shielding, a weakly conductive overhang corona shielding is applied and is electrically connected to the external corona shielding.
The external corona shielding of rotating electrical machines is subject to natural aging due to thermomechanical stresses, vibration or partial discharge activity, depending on the manufacturer and construction of the generator. As a result, part of the external corona shielding is eroded. A particular form in this context is erosion of electrical origin. The erosion locations affect in principle the entire length of the external corona shielding, wherein, depending on the manufacturer and construction of the generator, either the external corona shielding inside the laminated core or the external corona shielding outside the laminated core is affected to a greater degree. The erosion progressing in the axial direction disrupts the electrical connection between the overhang corona shielding and the laminated core. In the radial direction, there are on one hand partial discharges, and on the other hand the bars are loosened and thus, in extreme cases, subject to severe vibration.
There is therefore a need for a conductive substance which can bridge the existing (when used as a repair lacquer) or developing (when used during generator manufacture) gaps, and ensures conductive wetting of the exposed surface of the main insulation. However, this has the disadvantage that most polymer-based lacquers and paints shrink on drying or curing, and therefore in particular gap-free bridging of gaps is rendered more difficult. Thermomechanical load cycles also mean that re-occurrence of the gaps during operation cannot be excluded.